Melamine resins first appeared in Europe and later in America, a few years before World War II. Unlike phenolic resins, they have no objectionable odor, yet can be made into flexible to extremely hard lustrous surface coatings that are not easily stained. This is predominately attributed to the remarkable stability of the melamine's triazine ring after being cross-linked into an insoluble matrix. As a cross-linker, melamines have been introduced into molding, laminating, coating, gluing, and textile finishing.
One useful technology for producing microporous materials is thermally induced phase transition (TIPT). The TIPT process is based on the use of a polymer that is soluble in a diluent at an elevated temperature and insoluble in the diluent or forms a gel at a relatively lower temperature. The phase transition can involve a solid-liquid phase separation, a liquid-liquid phase separation or a liquid to gel phase transition. This technology has been employed in the preparation of microporous materials wherein a thermoplastic polymer and a diluent are separated by a liquid-liquid phase separation as described in U.S. Pat. Nos. 4,247,498 and 4,867,881. A solid-liquid phase separation has been described wherein the thermoplastic polymer on cooling crystallizes out. U.S. Pat. Nos. 4,539,256 (Shipman), 4,726,989 and 5,120,594 (Mrozinski) and 5,260,360 (Mrozinski et al.) describe such films containing a multiplicity of spaced, randomly dispersed, nonuniform shaped, equiaxed particles of a thermoplastic polymer, optionally coated with a liquid that is immiscible with the polymer at the crystallization temperature of the polymer. Microporous films made by a phase transition from a liquid to a gel have been described in U.S. Pat. No. 4,849,311. Micropores allow permeability to gases, including moisture vapor, but can be relatively impermeable to high surface tension liquids such as water.